6264 consistently larger than those of Kkin (Tables 1 and 11). For reactions of R + this difference could be due to the spectral measurements being made in highly acidic solutions, whereas the rate measurements are made in dilute alkali, but this explanation seems improbable because the kinetic salt effects due to the arenesulfonates are little affected by added 3 M NaCI, and differences between K , and Kkin are less for R+ than for MG+ (Tables I and 11). There are several explanations for the different values of K , and Kkin. (i) Our estimation of K , depends on the assumption that free carbocations are in equilibrium with one species of ion pairs, but there may be a family of ion pairs and the difference spectra may detect only those in which the ions are closely associated. (ii) In addition to the formation of relatively unreactive ion pairs there should be an unfavorable primary kinetic salt effect upon the reactions of oppositely charged ions, although the relatively small effects of salts having small anions, e.g., CI-, suggests that these primary effects are not (iii) The special positive salt effect upon S N 1 reactions in nonpolar solvents is explained in terms of an increased rate of dissociation of a solvent separated ion pair,' and if ionic recombination in water involves prior formation of an ion pair, e.g., MG+N(cf. ref 4), such a pair could be split unproductively by an arene sulfonate ion, e.g., Scheme 11, and this splitting should be more important in reactions of the less reactive MG+. Scheme I1 N-
MG'
a
MG'N-
-
MGN
MG'ArS03-
The carbocations, R+ and MG+, have delocalized charge, and they should be polarizable and relatively hydrophobic, and it is these properties which apparently dictate interactions with the aromatic solutes, irrespective of charge (Tables I and 11). For a given salt K , and Kkin come closer in magnitude as the binding with the carbocations increases. Acknowledgment. Support of this work by the National Science Foundation is gratefully acknowledged. References and Notes (1) S.Winstein, B. ADPSI. . . R . Baker, and A. Diaz, Chem. Soc.,Spec. Pub/.. No. 19, l(1965). (2)R . A. Sneen, Acc. Chem. Res., 8, 46 (1973).and references cited therein.
(3)J. M. Harris, Prog. Phys. Org. Chem., 11, 89 (1974). (4)C. D. Ritchie, ACC. Chem. Res.. 5 , 348 (1972);cf., C. D. Ritchie and P. 0. I. Virtanen, J. Am. Chem. Soc., 95,1882 (1973). (5) H. C. Brown and H. M. Bell, J. Am. Chem. SOC..88, 1473 (1966); M. M. Kreevoy and D. C. Johnson, Croat. Chem. Acta, 45,51 1 (1973). (6)M. J. Postle and P. A. H. Wyatt, J. Chem. Soc., Perkin Trans. 2, 474 (1972). 98, (7)C. A. Bunton and S. K. Huang, J. Am. Chem. Soc., 94, 3436 (1972); 515 (1974). (8)See R. H. Boyd in "Solute-Solvent Interactions". J. F. Coetzee and C. D. Ritchie. Ed., Marcel Dekker. New York. N.Y.. 1969,Chapter 3,for discussion of acidity functions.
(9)C. A. Bunton. J. H. Crabtree and L. Robinson, J. Am. Chem. Soc.. 90, 1258 (1968). (10)J. Gordon and R. L. Thorne. J. Phys. Chem., 73. 3643,3652 (1969);J. J. Brooks, W. Rhine, and G. D. Stucky, J. Am. Chem. SOC..94, 7346 (1972). (11) A. K. Colter, F. F. Guzik. and S. H. Hui, ibid., 88, 5754 (1966). (12)C. A. Bunton and S. K. Huang, J. Am. Chem. SOC..95,2701 (1973).
Clifford A. Bunton,* Sung K. Huang, Chang H. Paik Department of Chemistry, University of California Santa Barbara, California 93106 Received February 24, 1975 Journal of the American Chemical Society
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Conformation of the Tetrasaccharide Stachyose Sir:
Stachyose ( C ~ ~ H ~ ~ O ~ I - 0-a-D-galactopyranosylXH~O), ( 1 ---6)-O-a-galactopyranosyl-( 1+6)-O-a-D-glucopyranosyl ( 1-+2)-a-D-fructofuranoside,is a tetrasaccharide which
is widely distributed in the botahical world. It is usually concentrated in plant storage organs (roots, seeds, tubers, etc.), and is often found associated with sucrose and raffinose. These three sugars are the most common plant oligosaccharides. Raffinose is composed of a sucrose portion and one added galactopyranose residue. Stachyose contains an additional galactopyranose ring. Further sequential addition of galactose (1-6) linked residues leads to the relatively rare pentasaccharide, verbascose, and the hexasaccharide, ajugose, whose structures are undetermined.
f on
95
R R S R
s
R RI
Contained -7% of the cis isomer; yields are based upon the 2-methyl-2-oxazoline. bChlorofortn solutions. CEthyllithium was used as a 1.22 M solution in benzene-ether (70: 30); n-butyllithium was used as a 2.48 M solution in hexane; phenyllithium was used as a 1.53 M solution in benzene-ether (70:30). dRotations were measured on a Jasco DIP-180 automatic polarimeter at 25" in a l-cm3 (10 cm) cell. e Benzene was used as the solvent. f C . G. Overberger and I. Cho, J. Org Chern., 33,3321 (1968). 8 P. A. Levene and R. E. Marker, J. Biol. Chem., 95, 153 (1932). H. Rupe, Justus Liebigs Ann. Chem., 369, 31 1 (1909); configuration assigned by V. Prelog and H. Scherrer, Helv. Chirn. Acta., 42, 2227 (1959). iP. A. Levene and R. E. Marker,J. Biof. Chern., 115, 401 (1936); this is a calculated rotation value and unpublished results from our laboratory tend to indicate that the value is too high. I L. Lardicci, R. Menicagli, and P. Salvadori, Gazz. Chirn. Itnl.. 9 8 , 738 (1 968). k P. A . Levene and R . E. Marker,J. B i d Chern., 97, 5 6 3 (1932). /Configuration assigned as R ( - ) by CD curve comparison with R-(-)-3phenylpentanoic acid. Q
Journal of the American Chemical Society
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October 15, I975
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